The present invention relates generally to the transportation of natural gas and, more particularly, to a process for converting a multi-phase fluid stream containing a hydrocarbon gas and water to a stable gas hydrate slurry using a fluidized bed heat exchanger, wherein the resulting gas hydrate slurry is more suitable for transport.
Hydrocarbon gas, and particularly natural gas, produced in remote isolated onshore or offshore regions is often transported great distances to more populous regions for use. Pressurized pipelines or insulated oceangoing tankers are conventional means for transporting large quantities of natural gas. In many cases liquid phase water is mixed with the produced natural gas forming a wet natural gas. At the temperature and pressure conditions frequently encountered in gas pipelines, the natural gas and water react to form solid gas hydrates. The solid gas hydrates can occlude the gas pipeline by building up on the interior walls of the gas pipeline, ultimately aggregating into a plug or blockage.
The present invention recognizes a need for a cost-effective solution to the problem of solid gas hydrate formation in gas pipelines. As such, the present invention teaches a process which is designed to be practiced in advance of transporting a wet natural gas. The present process converts the wet natural gas to a gas hydrate slurry comprising solid gas hydrate particles suspended in a continuous liquid phase. Production of the gas hydrate slurry prior to pipeline transport of the wet natural gas precludes the accumulation of gas hydrates in the pipeline.
Several methods are known in the prior art for producing solid gas hydrates, but none are deemed satisfactory for the process of the present invention. For example, U.S. Pat. No. 5,536,893 to Gudmundsson teaches a method for producing gas hydrates in the form of a fluffy powder by spraying chilled liquid water into a cooled gas. PCT Patent Application WO9827033A1 to Heinemann et al. teaches a method for producing gas hydrates by adiabatically expanding a mixture of water and a cooled compressed gas across a nozzle to a lower pressure. Expansion of the mixture atomizes the water and produces solid gas hydrates. PCT Patent Application WO9919282A1 to Heinemann et al. teaches a method for producing gas hydrates in a fluidized bed reactor by conveying a gas phase upward to fluidize a bed of solid particles, while contacting the gas phase with a downwardly flowing chilled liquid water phase. All of the above-recited methods for producing gas hydrates are relatively inefficient because gas hydrate formation is an exothermic reaction and the evolution of latent heat in the reaction undesirably limits conversion. The above-recited methods require substantial preliminary sub-cooling of the feed streams or large adiabatic pressure drops, both of which substantially increase the cost and complexity of practicing the method.
Accordingly, it is an object of the present invention to provide an effective process for transporting a wet hydrocarbon gas, while substantially avoiding pipeline occlusion due to gas hydrate accumulation. More particularly, it is an object of the present invention to provide a process for efficiently converting a wet hydrocarbon gas to a gas hydrate slurry, wherein the gas hydrate slurry is suitable for transport. It is another object of the present invention to provide a process for converting a wet hydrocarbon gas to a gas hydrate slurry, wherein the conversion reaction is more nearly isothermal than in known conversion processes. It is another object of the present invention to provide a process for converting a wet hydrocarbon gas to a gas hydrate slurry, wherein the pressure losses associated with the conversion reaction are minimized. These objects and others are achieved in accordance with the invention described hereafter.
The present invention is a process for converting a multi-phase fluid stream to a gas hydrate slurry which is particularly suitable for transport via a motorized tanker transport vehicle or a pipeline. The multi-phase fluid stream contains an initial liquid phase comprising an initial water and an initial gas phase comprising a hydrocarbon gas. The process is initiated by entraining an abrasive inert solid particle medium in the multi-phase fluid stream to form a fluidizable mixture. The fluidizable mixture is conveyed through the interior of a heat transfer tube which is enclosed within a shell. A heat transfer medium resides within the shell, but is external to the heat transfer tube in fluid isolation from the fluidizable mixture. The wall of the heat transfer tube provides a heat transfer surface for heat exchange between the multi-phase fluid stream and the heat transfer medium which is cooler than the multi-phase fluid stream. When the multi-phase fluid stream contacts the tube wall, the multi-phase fluid stream is cooled to a temperature below the gas hydrate formation temperature. Consequently, at least a portion of the hydrocarbon gas and at least a portion of the initial water in the multi-phase fluid stream are converted to a plurality of solid gas hydrate particles. The solid particle medium substantially prevents accumulation of the solid gas hydrate particles on the tube wall, maintaining the plurality of solid gas hydrate particles in an unconsolidated condition within the fluidizable mixture. The fluidizable mixture, including the plurality of solid gas hydrate particles, is separated from the solid particle medium to recover a gas hydrate slurry. The gas hydrate slurry comprises a slurry solid phase suspended in a slurry liquid phase, wherein the slurry solid phase is the plurality of solid gas hydrate particles and the slurry liquid phase is the remaining portion of the initial liquid phase.
The remaining portion of the initial liquid phase contains one or more liquid phase components including a hydrocarbon liquid, an additive water, or an excess initial water. Where the initial water is the limiting reactant, the additive water may be added to multi-phase fluid stream before the conversion step as part of the initial liquid phase to convert substantially all of the hydrocarbon gas to solid gas hydrate particles. The additive water may also be added to the multi-phase fluid stream before, during or after the conversion step as part of the initial liquid phase for ultimate inclusion in the slurry liquid phase.
In accordance with a specific embodiment of the present invention, the process utilizes a fluidized bed heat exchanger having a shell enclosing a heat transfer medium flowpath, a fluidizable mixture flowpath in fluid isolation from the heat transfer medium flowpath, a heat transfer surface in heat communication with the heat transfer medium flowpath and fluidizable mixture flowpath, and an internal downcomer. The portion of the shell enclosing the heat transfer surface defines a heat transfer zone. The heat transfer medium is conveyed through the heat transfer medium flowpath to cool the heat transfer surface. The fluidizable mixture is simultaneously conveyed through the fluidizable mixture flowpath and is cooled upon contact with the heat transfer surface to form the plurality of solid gas hydrate particles. The fluidizable mixture, including the solid gas hydrate particles, is withdrawn from the heat transfer zone and the solid gas hydrate particles are separated from the solid particle medium to recover the gas hydrate slurry. The solid particle medium is returned to the heat transfer zone via the internal downcomer.
In accordance with an alternate specific embodiment of the present invention, the process utilizes a fluidized bed heat exchanger which is substantially the same as the above-described fluidized bed heat exchanger except that the present fluidized bed heat exchanger has an external separator and downcomer rather than an internal downcomer. Accordingly, the present fluidized bed heat exchanger operates in substantially the same manner as the above-recited embodiment except that the fluidizable mixture, including the solid gas hydrate particles, is conveyed to the external separator after withdrawal from the heat transfer zone. The solid gas hydrate particles are separated from the solid particle medium in the external separator to recover the gas hydrate slurry and the solid particle medium is returned to the heat transfer zone via the external downcomer.
In accordance with another alternate specific embodiment of the present invention, the process utilizes a fluidized bed heat exchanger which is substantially the same as the above-described fluidized bed heat exchangers except that the present fluidized bed heat exchanger lacks a downcomer and has heat transfer tubes extending upward into the separation zone of the fluidized bed heat exchanger. Accordingly, the present fluidized bed heat exchanger operates in substantially the same manner as the above-recited embodiments except that separation of the solid particle medium from the solid gas hydrate particles and remaining fluid components is performed in the upper portion of the heat transfer tubes. Thus, the solid particle medium does not recirculate, but remains at all times in the heat transfer tubes.
The invention will be further understood from the accompanying drawings and description.